This study describes cardiac pathologic findings in patients who received bone marrow transplantation for therapy of anemias, immune deficiency diseases or neoplasms. Some of the changes observed were related to complications of the underlying disease processes; others were side effects of the chemotherapeutic agents used in the treatment of neoplasms or in the process of immunosuppression, and others probably represented manifestations of abnormal immune mechanisms and of graft-versus-host disease.